Frame Build – Part 2
This part is going to cover
the build of the backbone, seat wishbones and rear wishbones. The seat wishbone
to backbone cut is difficult due to the steep angle involved. The tube in the
picture to the left has been marked for the angle (silver line). Note how the
cut starts from the front I.D. of the tube (if started from the OD of the tube,
a sharp edge would result and have to be sanded of flat again). The cut was
started in a drill press using a hole saw and tube-notcher. The hole saw bottoms out before the tube can
be cut through. Since the tube is round and the cut is also round, the cut does
not follow the marked angle. This excess material forward of the angle line
wraps and conformed to the backbones.
The picture on the left
shows how the excess material has been rough cut with a hacksaw (lower piece of
tube). The upper tube has been cleaned up by sanding a nice radius from the
front seat wishbone ID point to where the hole saw stopped cutting. Laying the
tube next to the backbone (center tube in picture) shows how the fit is
progressing. A 10” half round file works well to final fit the cope.
After the seat wishbones are
trimmed to fit the backbone, the rear ends are trimmed and coped to fit the
rear wishbones assembled in Part 1. The
parts can be assembled flat as shown to the left for a typical straight back
chopper frame. Or, the connections can be done at angles to create a drop seat
frame or dropped backbone for a drag style bike. By just changing he fitting of
the copes, a number of frame designs can be made from this simple set of bends.
In any case, it is a good idea to build a simple jig from wood to hold everything
in place for tack welding. Blocks of wood screwed to a sheet of MDF or Plywood
makes an adequate jig to hold everything in place.